Recently, development of a picture display device, termed a flat display device having a flat shape in distinction from a CRT display device, is briskly underway. This picture display device may be enumerated by a plasma display exploiting light emission from a phosphor element due to emission of ultraviolet rays by electrical discharge (referred to herein as PDP).
This PDP includes a pair of substrates arranged facing each other to define a gap in which an ionizable gas is enclosed. On the inner surface of one of the substrates is arranged the phosphor element. Electrical discharge is produced in the ionizable gas and a picture is displayed by light emission from the phosphor element by radiation of the ultraviolet rays produced by discharge.
This PDP is roughly classified into a so-called DC type PDP in which discharge electrodes are arranged on both the substrate provided with the phosphor element and the substrate not provided with the phosphor element for facing each other in order for the electrical discharge to take place along the thickness, and a so-called AC type PDP in which the discharge electrode is provided only on the substrate not carrying the phosphor element and the discharge electrode is covered by a dielectric layer in order for the electrical discharge to take place in the in-plane direction.
As a flat type display, other than the PDP, a picture display device of the type in which a liquid crystal layer having an enclosed liquid crystal is driven in accordance with a so-called active matrix system of driving an active element, such as a transistor, provided from pixel to pixel (referred to hereinafter as a TFT liquid crystal display) is stirring up notice.
However, with the TFT liquid crystal display, since it is necessary to provide a large number of semiconductor devices, such as transistors, there is raised a problem of production yield in case a display of a large area is used for forming a large-sized screen, thus raising production cost.
For solving these problems, there has also been proposed a picture forming device operating under a system in which discharge plasma is used in place of MOS transistors or thin-film transistors as active elements.
This type of the picture display device may be such a device having a display panel which is obtained on superposing a plasma cell having plural discharge electrodes for plasma discharge with a second substrate having electrodes substantially at right angles to the discharge electrodes via a liquid crystal layer of a liquid crystal as an electro-optical material.
The plasma cell includes a first substrate, having plural substantially parallel discharge electrodes on its major surface and a thin sheet of a dielectric material at a pre-set separation from the first substrate. An ionizable gas is sealed in the space between the first substrate and the thin dielectric sheet and the peripheral portion of the resulting assembly is sealed with a sealant. This plasma cell is divided by partitioning wall sections into plural line-shaped plasma chambers in which plasma discharge can be produced.
The second substrate has plural electrodes extending substantially at right angles to the discharge electrodes of the plasma cell on its major surface. This second substrate is superimposed via liquid crystal layer on the dielectric plasma sheet of the plasma cell with the electrode carrying surface as the facing side.
In this display panel, the liquid crystal is driven by sequentially switching scanning the plasma chambers of the plasma cell and by applying a signal voltage across the electrodes of the second substrate facing the plasma cell with the interposition of the liquid crystal layer in synchronism with the switching scanning, with the portions of the plasma chambers intersecting with the electrodes of the second substrate delimiting pixels.
In both the PDP and the picture display device in which the liquid crystal layer is driven by discharge plasma, it is necessary for an ionizable gas to be enclosed between the facing substrate pairs or between the first substrate and the thin dielectric sheet, as described above. This ionizable gas is sealed by boring a through-hole in the substrate, inserting a glass tube therein, evacuating the space via this glass tube, charging the gas into the space and sealing the glass tube. This sealing of the glass tube is by heating the glass tube, radially compressing the glass tube by external pressure under heating and burning off the outer portion for allowing the end to be sealed spontaneously, or by compressing the tube to a bar and cutting off its distal end.
Recently, the sealed gas tends to be compressed to a higher pressure. Specifically, while the gas was sealed hitherto under vacuum or under a low vacuum less than one atmosphere, the pressure in the vicinity of one atmosphere or a higher pressure is used in sealing the gas.
If such higher pressure is used, the pressure in the tube is higher than the external pressure, that is atmospheric pressure, the glass tube becomes difficult to seal with the conventional method. That is, if the glass tube is heated for sealing, the glass becomes softened or melted to raise the internal pressure to a value higher than the external pressure. Thus, the heated portion is expanded like a balloon and explodes to render sealing impossible.
In the manufacture of a tube bulb, a tube of oxygen-free copper is evacuated and charged with a gas until the gas pressure is equal to or higher than the external pressure. After charging the gas, the end of the tube bulb is pinched off for sealing. This method is not desirable in view of increased production cost and lowered productivity brought about by the difficulty in mounting the tube of oxygen-free tube in position.
Thus, for sealing a glass tube of a picture display device in which the gas is sealed under a higher sealing pressure, it may be envisaged to raise the external pressure to higher than one atmosphere for apparently lowering the pressure in the glass tube for sealing the glass tube, or to raise the temperature of the glass tube to close to the glass softening temperature for pinching off the distal end of the tube.
However, with the former method, the method of sealing the glass tube is limited to heat-sealing by a heater wire or high frequency heating sealing. Since it is necessary in this case to seal the portion around the sealing point hermetically, thus increasing the size of the sealing device. In addition, the sealing needs to be performed individually from one glass tube to another, thus affecting mass producibility or productivity.
With the latter method, the glass tube needs to be increased in tube thickness because the tube tends to be expanded if it is reduced in wall thickness. In this case, the glass tube is heated and compressed gradually and pinched off ultimately. This pinch-off process affects the reliability of the picture display device such that operational reliability of the resulting product cannot be assured. In addition, the pinch-off process frequently leads to impact applied to the paired substrates or to the first substrate and the thin dielectric sheet, such that the connecting portion between the glass tube and the substrate formed of a sealant material is subjected to peeling to affect the reliability.
Thus, in the manufacturing method for the picture display device, it is incumbent to render it possible to provide a picture display device capable of coping with the increased sealant gas pressure in order to improve mass-producibility and operational reliability of the picture display device.